During endochondral bone development, chondrocytes of the epiphyseal growth-plate exhibit major changes in morphology, energy metabolism, and gene expression and matrix biosynthesis. The long-term objective of this application is to understand the mechanisms underlying the maturation and development of the growth plate chondrocyte, specifically the process of cellular hypertrophy. Based on our recent observations, we postulate that changes in cellular energy metabolism of the chondrocyte, mediated by creatine kinase (CK) and the creatine phosphate (CP) circuit, are functionally involved in cellular hypertrophy. Experiments are proposed to test the hypothesis that depletion of CP and the accompanying fall in chondrocyte energy reserves serve as an initiating factor for the subsequent development of cellular hypertrophy. Specifically, we postulate that the functional state of the CK/CP energy-generating system is responsible, via changes in cellular Na+ handling, for the characteristic increase in cell size marking chondrocyte hypertrophy. Four specific aims are proposed: l) to define the energetic state of cells in the growth cartilage in situ in relationship to the CP circuit, and the development of hypertrophy; 2) to define CK activity and the CP circuit in cultured chondrocytes in vitro, and to determine how changes in these parameters affect chondrocyte energy metabolism and functional activities associated with the hypertrophic state; 3) to modulate the activity of components of the CK/CP system using chemical agents, vitamin D, and molecular biology techniques, and to determine the effects of these changes on chondrocyte energy state and hypertrophy; and 4) to measure [Na]i in growth plate chondrocytes in situ and in culture, and to relate changes in [Na]i and cellular Na+ handling to cell hypertrophy and the activity of the CK/CP system. Results from the proposed studies should further our understanding of the biology of the growth plate, bone and cartilage, and connective tissues in general, as related to skeletal growth and development. Useful directions will also be gained for future examination of the possible involvement and/or diagnostic importance of CK expression and isozyme profiles in various human and animal skeletal diseases, such as osteopetrosis, osteoporosis, and achondroplasia, etc.